In Situ-Generated Volatile Precursor for CVD Growth of a Semimetallic

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Functional Inorganic Materials and Devices

In-situ Generated Volatile Precursor for CVD Growth of a Semimetallic 2D Dichalcogenide Zhenfei Gao, Qingqing Ji, Pin-Chun Shen, Yimo Han, Wei Sun Leong, Nannan Mao, Lin Zhou, Cong Su, Jin Niu, Xiang Ji, Mahomed Mehdi Goulamaly, David A. Muller, Yongfeng Li, and Jing Kong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13428 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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ACS Applied Materials & Interfaces

In-situ Generated Volatile Precursor for CVD Growth of a Semimetallic 2D Dichalcogenide ∥

Zhenfei Gao†,‡, Qingqing Ji*,†, Pin-Chun Shen§, Yimo Han , Wei Sun Leong§, Nannan Mao§, Lin Zhou§, Cong Su†, ⊥ , Jin Niu†, Xiang Ji§, Mahomed Mehdi Goulamaly§, David A. Muller ∥ ,#, Yongfeng Li,*, ‡ and Jing Kong*,†,§ †

Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA

02139, USA. ‡

State Key Laboratory of Heavy Oil Processing,China University of Petroleum, Beijing 102249,

P.R. China. §

Department of Electrical Engineering and Computer Science, Massachusetts Institute of

Technology, Cambridge, MA 02139, USA. ∥

School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14850,

USA. ⊥

Department of Nuclear Science and Engineering, Massachusetts Institute of Technology,

Cambridge, MA 02139, USA. #

Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY 14853, USA.

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ABSTRACT Semimetallic layered transition metal dichalcogenides, such as TiS2, can serve as a platform material for exploring novel physics modulated by dimensionality, as well as for developing versatile applications in electronics and thermoelectrics. However, controlled synthesis of ultrathin TiS2 in a dry-chemistry way has yet to be realized because of the high oxophilicity of active Ti precursors. Here we report an ambient pressure chemical vapor deposition (CVD) method to grow large-size, highly crystalline 2D TiS2 nanosheets through insitu generating titanium chloride as the gaseous precursor. The addition of NH4Cl promoter can react with Ti powders and switch the solid-phase sulfurization reaction into a CVD process, thus enabling the controllability over the size, shape, and thickness of the TiS2 nanosheets via tuning the synthesis conditions. Interestingly, this semimetallic 2D material exhibits near-infrared surface plasmon resonance absorption and a memristor-like electrical behavior, both holding promise for further application developments. Our method hence opens a new avenue for the CVD growth of 2D metal dichalcogenides directly from metal powders and pave the way for exploring their intriguing properties and applications.

KEYWORDS: Chemical vapor deposition, semimetallic 2D materials, titanium disulfide, transition metal dichalcogenides, chloride promoter.

INTRODUCTION Titanium-based dichalcogenides (TiS2, TiSe2 and TiTe2) are often referred as semimetals with vanishing indirect bandgaps.1-4 They have been established as promising excitonic materials5 that can facilitate Bose condensation,6 a macroscopic quantum phenomenon closely related to superfluidity and superconductivity. In their two-dimensional (2D) forms, the diverse electronic


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phases addressable by external electric fields can bring about insightful understandings on the origin and interplay of these many-body states.7 Being semimetallic, the carrier concentration and electrical conductivity in these layered materials can be facilely engineered through ion intercalation, followed with greatly improved thermoelectric figure of merits.8-9 In addition, the localized surface plasmon resonances (LSPRs) hosted by these materials are in the near-infrared range,10 which are potentially useful for biological imaging,11 photothermal therapy,12 and optical communication applications.13 These remarkable properties have made Ti-based dichalcogenides an indispensable building block in the 2D materials toolbox, as in supplement to those metallic and semiconducting transition metal dichalcogenides (TMDs).14-17 Although dimensionality effects have been predicted theoretically in tuning the electronic/optoelectronic

properties

of

the

Ti-based

dichalcogenides,18-19

experimental

explorations have been rarely reported because of the sluggish development in synthesizing the 2D materials. Chemical vapor transport (CVT), as a universal technique for bulk TMD synthesis,20 is not only time-consuming but also involves complicated processes that have indirect access to corresponding 2D crystals. Following this route a redesigned CVT process was lately developed for synthesizing 2D TiSe2 nanosheets,21 the key of which is using a less-active transport agent, together with a much-reduced growth duration of several minutes. Nevertheless, several challenges remain to be addressed such as the process complexity involving vacuum ampule sealing, and the residue of transport agents that possibly contaminate sample surfaces. In addition, wet-chemistry methods such as chemical exfoliation22 and solution reaction10 were also utilized for producing TiS2 nanosheets with, however, limited lateral sizes (< 1 µm) and uncontrolled thicknesses (1-100 nm).


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On the other hand, chemical vapor deposition (CVD) as a dry-chemistry technique has been well recognized as an effective way to synthesize high-quality 2D molybdenum and tungsten dichalcogenides.23-24 The CVD process typically involves the reaction of volatile MoO3 or WO3 with chalcogen vapors that provides superior controllability over the sheet thickness and uniformity. Nevertheless, this synthesis strategy cannot be extrapolated directly to the growth of Ti dichalcogenides, because of the nonvolatility of high-melting-point TiO2 at typical CVD temperatures. An alternative way is using volatile metal chloride precursors, which has been successfully applied for the CVD growth of 2D VS2 and TaS2 nanosheets.25-26 However, these active metal chlorides, in particular TiCl4, are prone to hydrolysis in ambient conditions, making oxygen an inevitable dopant in the resulted TMD samples and possibly destructing their intrinsic properties. To date, it remains a great challenge to achieve CVD growth of high-quality 2D titanium dichalcogenides with large domain size, monolayer thickness, and large-scale uniformity. To address the above issues, we herein report a CVD strategy that involves the in-situ reaction of titanium powders and chalcogen vapors for the growth of Ti dichalcogenide nanosheets. Ammonium chloride (NH4Cl) was employed as the assistive reagent that greatly promoted evaporation of the high-melting-point Ti powders by forming volatile chloride species for the CVD process. Using this method, monolayer TiS2 has been produced successfully whose shape, size, and thickness can be finely adjusted by tuning the growth conditions as detailed below. It is anticipated that the described method here is not only limited to the production of monolayer TiS2, but also can be generalized as a universal strategy for synthesizing various TMD materials, under the condition that the metal powders react with hydrogen chloride vapors.

EXPERIMENTAL SECTION ACS Paragon Plus Environment

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Materials. Titanium powder (99.98%, trace metal basis) and sulfur powder (≥99.5% purity) were purchased from Sigma Aldrich. Ammonium chloride (99.999%, metal basis) was purchased from Alfa Aesar. Mica was purchased from Ted Pella and was mechanically cleaved with fresh surface for the CVD growth. CVD Growth of TiS2 Nanosheets and Films. As a pretreatment of dehydration, Ti/NH4Cl mixture (~100 mg, mass ratio of 1:5) was dispersed in acetone and shaken for 5 min. The hydration water in NH4Cl was effectively extracted by acetone because of their miscibility. After the mixture powder was settled down, the upper supernate was discarded, and the mixture was quickly loaded into the CVD system before its drying up. The remnant acetone soon evaporated completely after the furnace heated up, thus exerting no influence on the CVD growth of TiS2. A three-temperature-zone furnace equipped with a 1-in.-diameter quartz tube was used for the CVD growth. Two heat-insulating plates were placed at the junctions of neighboring zones to suppress heat convection so that the set temperature in each zone can keep stable during growth. Sulfur powder (~2 g) in an alumina boat and Ti/NH4Cl powder were placed in the upstream and midstream zones set at 250 °C and 225 °C, respectively. Mica substrates for the CVD growth were put in the downstream zone set at 450 °C. Prior to growth, the CVD system was purged with 1000 sccm Ar/H2 (volume ratio of 95:5) gas flow for 3 min to eliminate oxygen residues. In the ramping period (t< 20 min), the Ar/H2 carrier gas was kept at a low flow rate (50 sccm). When the growth temperature reached 450 °C, the carrier gas flow was changed to 350 sccm to effectively transport the Ti precursor for the CVD growth. The growth lasted for 10 min and the furnace was cooled down naturally under an Ar/H2 gas flow of 150 sccm. Characterization. Characterization was implemented using optical microscopy (Axio Imager, Carl Zeiss), Raman (Witec CRM 200 Confocal Raman Microscopy), UV/Vis/NIR (Perkin Elmer


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Lambda 1050), AFM (Dimension 3100, Veeco Instruments Inc.), XPS and TEM. The excitation wavelength for the Raman measurement is 532.5 nm. XPS measurements were carried out using PHI Versaprobe II with Al Kα monochromated (1486.6 eV) at a pass energy of 23.5 eV and 187.85 eV for high resolution and survey spectra, respectively. The highest peak in the C 1s spectra was shifted to 284.8 eV for charge correction. The CARL ZEISS: MERLIN™ scanning electron microscope was used to identify the specimen on the SiO2/Si surface, which is operated at a voltage of 20 kV. ADF-STEM images were taken with a FEI TITAN operated at 120 kV. The beam convergence angle was 25 mrad, with a probe current of ~10 pA.

RESULTS AND DISCUSSION The CVD growth was implemented in a three-temperature-zone furnace as illustrated in Figure 1a. Sulfur, dehydrated Ti/NH4Cl powders, and mica substrates were placed in individual zones from upstream to downstream to achieve rigorous control of the precursor evaporation and the CVD reaction (see Experimental Section for more details). The use of NH4Cl was found to be critical for evaporation of the high-melting-point Ti powder at a moderate temperature of < 450°C. As a control experiment, no TiS2 growth was observed on the downstream substrates with only Ti powder and sulfur vapor (illustration in the region outlined by the blue dash line in Figure 1b). Considering that NH4Cl decomposes into NH3(g) and HCl(g) at above 300 °C, and Ti reacts with HCl gas to form volatile TiClx species, the CVD process after NH4Cl incorporation can be understood as follows, 2Ti(s) + 2xHCl(g) → 2TiClx(g) + xH2(g) 2TiClx(g) + xH2(g) + 4S(g) → 2TiS2(s) + 2xHCl(g)

(1) (2)


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By combining the two chemical equations, we get the following reaction Ti(s) + 2S(g) → TiS2(s)

(3)

Therefore, the addition of NH4Cl could readily transform this solid-phase sulfurization reaction (3) into a CVD process through in-situ generation of volatile TiClx species (Figure 1b). A typical optical microscope (OM) image of as-grown TiS2 on mica substrates is displayed in Figure 1c, showing triangular-shaped nanoflakes over the whole substrate surface that highlights the desired vapor deposition behavior.

Figure 1. Synthesis of TiS2 nanosheets. (a) Experimental setup for the CVD growth of TiS2 nanosheets on mica substrates. (b) Schematic illustration of the CVD growth mechanism with NH4Cl promoters. (c) Typical OM image of TiS2 nanosheets grown on mica. Interestingly, the morphologies of TiS2 nanosheets grown at 450 °C were found to strongly depend on the used substrates. Two typical growth patterns are schematically illustrated in Figure 2a, in which half-hexagonal nanosheets tend to grow vertically on SiO2/Si, while


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triangular or truncated triangular flakes evolve on mica, demonstrating an in-plane growth mode. Such vertical or horizontal growth behavior can be tentatively understood in terms of the surface wettability of the substrates by the TiS2 layers, and/or the reaction dynamics of the in-situ generated TiClx precursors.27 Mica in our case, having a high-energy ionic surface, can be reasonably expected to induce the adherent TiS2 growth, similar to the growth of a variety of 2D layered materials on this substrate.28-31 In contrast to the sparse distribution of individual flakes on SiO2/Si (Figure 2b), TiS2 growth on mica can evolve into continuous films up to centimeter scale. Shown in Figure 2c is an ultrathin TiS2 film having nearly full coverage on mica outside those thick flakes, whose thickness was further confirmed by atomic force microscopy (AFM) to be ~0.6 nm, that is, a monolayer. In both cases, the obtained TiS2 nanosheets exhibit flat surfaces and good foldability (Figure 2e, f), indicative of their 2D nature and decent stability in ambient conditions. At elevated temperatures of ≥ 600 °C, TiS2 grown on both substrates evolve into discrete hexagonal flakes of greater thicknesses (Figure 2d, f and Figure S1). Growth at such temperatures is hence inferred to be governed by the energetics of TiS2 itself, rather than mediated by the substrates. TiS2 growth on mica is more interesting because of the preferable film-forming property. The surface morphologies of TiS2 on this substrate can be finely controlled by adjusting the growth time (tgr). When tgr was shorter than 5 min, no TiS2 triangles formed on mica, indicating a dead time of > 5 min for precursor transport and TiS2 formation. When tgr = 7 min, discrete TiS2 flakes of small sizes started to appear on mica (Figure S2a). Notably, the thicknesses of these individual flakes varied with each other, suggesting an island growth behavior rather than a layer-by-layer mode. When tgr increased to 10 min, large TiS2 flakes with thicknesses of tens of nanometers formed on mica, which were stitched together by monolayer TiS2 films in the


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surrounding regions (Figure S2b and Figure 2c). The deposition amount can be further increased by prolonging the growth time or elevating the evaporation temperature of the Ti/NH4Cl mixture. These results demonstrate the controllability of the CVD process with the assistance of NH4Cl promoters.

Figure 2. Microscopic characterizations of the TiS2 nanosheets. (a) Schematic illustration of two growth patterns for TiS2 crystals on different substrates. (b) OM image of the half-hexagonalshaped TiS2 nanosheets grown on SiO2/Si substrate after the contact transfer process. (c, d) OM images of triangular and hexagonal TiS2 grown on mica substrates at 450 °C and 600 °C, respectively. (e-g) AFM height images of different-shaped TiS2 shown in b, c and d, respectively. Insets are the corresponding height line profiles.


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While the vertical TiS2 nanosheets grown on SiO2/Si can be transferred onto other substrates using the contact transfer technique (Figure S3) as reported for CVD-VS2,25 TiS2 flakes and films grown on mica can also be facilely transferred with the aid of polymer support on top and ammonium fluoride solution (~0.1 g/mL) as the mica etchant. Figure S2c is an OM image of the TiS2 nanosheets transferred from mica onto SiO2/Si, exhibiting well-preserved morphologies as that on mica substrates (Figure S2a). Surprisingly, Raman spectra collected on the transferred samples show identical line shapes as that of as-grown TiS2 on mica (Figure S2d) without any emergence of TiO2 signals. This demonstrates that, high crystal quality TiS2 nanosheets are resistive to water hydrolysis to some extent at room temperature, hence surviving the wet chemistry of the transfer process. The transferability of TiS2 grown on mica provides new possibilities that this semimetallic 2D material can be integrated with other layered materials to form functional van der Waals architectures,32-33 thus broadening the choice of candidates in the 2D materials toolbox. Figure 3a and b are the scanning electron microscope (SEM) and the bright-field transmission electron microscope (TEM) images, respectively, of the transferred TiS2 nanosheets, both morphologies indicating the flexibility and 2D nature of the TiS2 sample at such thickness. Figure 3c displays a high-angle annular dark-field (HAADF) image of the TiS2 nanosheets taken by scanning transmission electron microscope (STEM), in which the Ti atoms, having the greater atomic number show higher contrast, while the S atoms show lower contrast. In the zoomed-in STEM image (Figure 3d), each Ti atom is surrounded by six sulfur atoms, indicating the hexagonal lattice structure. Corresponding diffractogram in Figure 3e also shows the characteristic hexagonally arranged spots. This atomic arrangement is consistent with the typical 1T phase structure as displayed in Figure 3f. Interestingly, line profiles extracted from Figure 3c


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exhibit higher Ti:S intensity ratios than the theoretical value (Figure S4), which possibly indicates the existence of sulfur vacancies in the 2D crystal.

Figure 3. SEM and TEM characterizations of the crystal structure of the TiS2 nanosheets. (a) SEM image of a TiS2 nanosheet on SiO2/Si. (b) Large field-of-view TEM image of a TiS2 nanosheet transferred on lacey carbon films supported by Cu grids. (c) HAADF-STEM image of TiS2 nanosheets. (d) A zoomed-in image of the area highlighted in c. (e) Corresponding diffractogram of d. (f) Schematic of the atomic structure of 1T-TiS2 in top view and side view. Raman spectroscopy with a 532 nm excitation laser was further utilized to characterize the crystal quality of the obtained TiS2 nanosheets. In Figure 4a, the synthesized TiS2 nanosheet shows two Raman peaks at ~230 cm-1 and ~332 cm-1, corresponding to the in-plane Eg and outof-plane A1g modes of 1T-TiS2.34 Figure 4b is the Raman mapping image with the strongest A1g peak of a half-hexagonal TiS2 nanosheet on SiO2/Si, highlighting the thickness uniformity of the sample. The thickness-dependent Raman evolution of the TiS2 flakes was also presented as


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Figure S5 in Supporting Information, consistent with previous reports.35 UV/Vis/NIR absorption spectra were also collected for TiS2/mica samples having large portions of thick and thin flakes (Figure 1c and 2c, respectively). An interesting feature of the absorption spectra (Figure 4c) is the LSPR peaks at ~1200 nm, which provides a direct evidence for the semimetallic property of our 2D TiS2 samples.10 By using X-ray photoelectron spectroscopy (XPS), the elemental composition and bonding types of thick TiS2 films can be conveniently determined. The Ti 2p peaks at 456 eV (2p 3/2) and 462 eV (2p 1/2) were assigned to Ti-S bonds in Figure 4d, while the satellite peaks with a weight of less than 20% were confirmed to be Ti-O bonds.36 Figure 4e is the XPS spectrum of sulfur for the TiS2 sample that reveals the presence of both elemental and anionic states. Using the deconvoluted intensities, the Ti4+:S2- atomic ratio after excluding those Ti atoms bonded with oxygen was revealed to be 1:1.9, in good agreement with the chemical formula of TiS2. We note that all the above characterization results are based on TiS2 samples grown with dehydrated NH4Cl powders (see Experimental Section for more details). The hygroscopicity of NH4Cl was found to be detrimental to the successful growth of high-quality TiS2. Using NH4Cl promoters that were kept in air and slightly hydrated by nature, the resulted TiS2 nanosheets demonstrate deteriorated morphologies and crystal qualities both on mica and SiO2/Si substrates (Figure S6). XPS characterization (Figure 4f) shows that the Ti elements in this case are primarily associated with TiO2 rather than TiS2 (~75% versus ~25%), and the emerging Raman peak at ~150 cm-1 (Figure S6c), characteristic for anatase TiO2,37 also indicates mixed phases of TiO2 and TiS2 in the final products. These results are schematically illustrated in Figure 4g, emphasizing a moisture- and oxygen-free environment for the growth of chemically pure TiS2 crystals. Our growth strategy by in-situ generating volatile metal chlorides eliminates their


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chance of hydrolysis, promising for the synthesis of a variety of moisture- and oxygen-sensitive 2D TMD materials, such as ZrS2, TaS2, and MoTe2. Moreover, compared with the NaClpromoted TiS2 growth,38 the NH4Cl promoter decomposes completely into NH3 and HCl, thus leaving no condensed solid on sample surfaces, and the reaction between Ti and HCl vapor is efficient at temperature as low as ~300 °C. These features make the NH4Cl-promoted CVD a useful alternative for producing clean 2D TiS2 crystals.

Figure 4. Spectroscopic characterizations of as-grown TiS2 nanosheets. (a) Raman spectrum of TiS2 nanosheets grown on SiO2/Si. (b) Corresponding Raman intensity mapping of the A1g peak. (c) UV/Vis/NIR absorption spectra of the as-grown samples having large portions of thick and thin TiS2 flakes (red and blue curves, respectively). Black dash lines are imposed for eye guidance. (d) XPS spectrum of Ti in as-grown samples using dehydrated NH4Cl promoter. (e)


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XPS spectrum of sulfur in the TiS2 samples. (f) XPS signal of Ti in as-grown samples using hydrated NH4Cl promoter. (g) Schematic illustration of the CVD growth in moisture-free and moisture-bearing environments. In addition, electrical measurements were performed on the as-grown TiS2/mica samples. The Ti/Au electrodes were defined directly on the TiS2 using photolithography and e-beam metal deposition. Figure 5a exhibits the OM image of a fabricated device with a channel width (W) of 50 µm and channel length (L) of 5 µm for adjacent electrodes. The corresponding current-voltage (I-V) curve in Figure 5b demonstrates an ohmic contact at the Ti-TiS2 interface and a remarkable conductivity of ~4.5 mS for the device. The derived sheet resistance is 2.2 kΩ/sq using W = 50 µm and L = 5 µm. Multiple devices were measured with their sheet resistances distributed in the range of 2-5 kΩ/sq. Using the characteristic flake thickness of ~50 nm, this can be translated into the bulk resistivity on the order of 10-4 Ω·m, a typical value for a semimetallic material. Very interestingly, the TiS2 flakes manifest a memristor-like behavior in a wider sweep range, with the ohmic contacts persisting both in the high- and low-resistance states (Figure 5c). For those TiS2 flakes transferred on SiO2/Si, electrodes were deposited using electron-beam lithography (EBL). Subsequent electrical measurements revealed non-Ohmic contacts and higher resistivities (Figure S7), which is probably associated with interfacial oxide barriers generated during the nanofabrication processes.


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Figure 5. Electrical measurement of the TiS2 nanosheets. (a) OM image of a microdevice fabricated on as-grown TiS2/mica. (b) I-V curve of the TiS2 nanoflake in (a). (c) Memristive-like behavior of a typical TiS2 flake in a wider sweep range along 1→2→3→4.

CONCLUSION In summary, we have developed a NH4Cl-assisted ambient pressure CVD method for the controlled synthesis of a 2D semimetallic dichalcogenide, TiS2. Our method provides direct access to highly crystalline TiS2 nanosheets and films, with superior controllability over the growth morphologies such as the domain size, shape, and thickness. The semimetallic nanosheets, as manifested by the near-infrared LSPR absorption and electrical measurements, hold promise for exploring a variety of intriguing physics and applications. More importantly, our growth strategy of in-situ generating volatile metal chlorides can be useful for synthesizing those moisture- and oxygen-sensitive 2D TMD materials. As a preliminary demonstration, this synthesis methodology has been extended for the growth of another two chalcogenides (Figure S8), whose evaporation and growth temperatures are distinctly lower than other thermal growth methods, yet without degrading the crystal quality. Our study hence suggests a new and promising CVD paradigm for versatile 2D atomic crystals and their heterostructures. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental details and supporting figures (PDF)


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J.K.). *E-mail: [email protected] (Q.J.). *E-mail: [email protected] (Y.L.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Z.G. and Y.L. acknowledge support from the National Natural Science Foundation of China (Nos. 21576289, 21776308), Science Foundation of China University of Petroleum, Beijing (No. C201603) and Thousand Talents Program. Q.J. and L.Z. acknowledge support by the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319. P.C.S. acknowledges support from the Center for Energy Efficient Electronics Science (NSF Award 0939514). W.S.L. acknowledges support from SUTD-MIT Postdoctoral Fellows Program and the Air Force Office of Scientific Research under the MURI-FATE program, Grant No. FA9550-15-1-0514. N.M. acknowledges support from NSF Grant 2DARE (EFRI-1542815) and DMR-1507806. C.S. acknowledges support from U.S. Army Research Office through the MIT Institute for Soldier Nanotechnologies (Grant No. 023674). Y.H. and D.A.M. acknowledge Cornell Center for Materials Research with funding from the NSF MRSEC program (DMR-1719875).


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